This invention relates to an open architecture superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called xe2x80x9cMRIxe2x80x9d), and more particularly to a magnet assembly utilizing a conical or prism shaped imaging region and suitable for interventional surgical procedures.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that when a power source is initially connected to the magnet coils for a period (for example, of only ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of electrical resistance at the superconducting temperature, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
A problem encountered in the use of most MRI equipment is that they utilize solenoidal magnets enclosed in cylindrical structures with a central bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm central bore, which can induce claustrophobia in some patients. Moreover, there has been a desire to utilize MRI for interventional diagnostic or surgical procedures such that the surgeon can observe the imaging during the procedure. The desirability of an open architecture MRI magnet in which the patient is not essentially totally enclosed has thus long been recognized. Unfortunately, an open architecture MRI magnet to provide open space about the patient poses a number of additional and unique technical problems and challenges. One problem is to provide a suitable superconducting structure which will provide the required magnetic field yet occupies much less space than conventional cylindrical MRI magnet structures, and yet which nevertheless can provide the required strong and homogeneous magnetic field imaging region or imaging volume.
Existing open architecture designs include double donut and double iron pole pieces with intervening support members with the patient positioned between the donuts. However, while providing patient access, such a design limits surgeon access because of the required supports and double magnetic members. As a result access to the patient is limited in such designs which are not as amenable to medical interventional and surgical procedures as desired.
The constraints on mechanical and magnetic subassemblies in a superconducting magnet are driven by the need to provide a suitable homogeneous and strong imaging region usually midway between the magnetic members in which to do the actual MRI imaging. A typical MRI imaging region is a sphere 15 to 20 inches in diameter, or a cylindrical imaging volume 10 to 15 inches in diameter and 10 to 20 inches long. The use of symmetrical or mirror image magnetic components about the imaging volume with magnetic shaping and positionable shimming members have enabled the adjustment or fine tuning of the resultant magnetic field to provide a spherical or cylindrical volume imaging volume with the required magnetic homogeneity.
For interventional procedures, it has been determined that a spherical or cylindrical imaging region or volume is not necessary. A smaller imaging or viewing region at the point where the surgical procedure or needle insertion begins on the surface of the body of the patient can readily utilize a smaller imaging volume which is not a spherical or cylindrical volume. It may even be desirable for such procedures to have the imaging volume increase as it penetrates into the body away from the point where the surgery begins, that is a generally conical imaging volume. Moreover., it is highly desirable and often necessary to provide a flexible design which facilitates construction while minimizing cost of the MRI equipment.
It is thus desirable to provide an open architecture MRI superconducting magnet in which the patient is as fully accessible to the physician as possible and which includes a spherical or prismatic imaging volume.
In accordance with one form of the invention an open architecture platform superconducting magnet suitable for interventional procedures includes shaped magnetic yoke and associated magnet coil. The yoke and coil are shaped to provide an imaging region in open space above the coil which is tapered, in that the cross section of the imaging volume decreases in the direction remote from the coil. The tapered imaging region is in the shape of a frustrum of a right cone or a prism. A patient support is positioned above the magnet coil and magnet coil axis. The magnetic field may have a monotonic gradient.